1. Field of the Invention
The present invention relates to a device isolation method of a semiconductor memory device and flash memory device fabricating method using the same, by which characteristics of the semiconductor memory device can be enhanced.
2. Discussion of the Related Art
Generally, in forming floating gates for fabricating a flash memory device, it is important to electrically separate the floating gates in neighbor cells.
A floating gate is formed in a manner of forming a device isolation layer by shallow trench isolation (hereinafter abbreviated STI), forming a tunnel oxide layer by depositing an oxide layer over a substrate, depositing a doped polysilicon layer on the tunnel oxide layer, and patterning the doped polysilicon layer.
Yet, in the related art floating gate forming method, it is difficult to precisely pattern the floating gate due to the misalignment between the device isolation layer and the etch mask for patterning the floating gate, i.e., the photoresist pattern. Hence, limitation is put on reducing a cell size.
To overcome such a problem, there is a method of forming a tunnel oxide layer and a floating gate after etching an upper part of a device isolation layer having a positive profile.
However, since a lateral side of the floating gate exposed by etching the device isolation has a negative profile, the polysilicon layer is not completely removed in patterning a control gate, whereby a bridge occurs between control gates in neighbor cells.